The broad goal of this work is to learn how gene expression is controlled in myeloma cells and how the genetic program at this terminal stage of differentiation is established over the course of B lymphocyte development. A specific goal of the present proposal is to determine how the immunoglobulin heavy chain gene enhancer operates in early (pre-B) and late-stage (myeloma) B lymphocytes. We will also use analysis of a spontaneously-derived myeloma variant (LP1) as a means for exploring how post-transcriptional processes contribute to stage-specific differences in IgH gene expression, as well. Our previous studies have shown that the constitutive, high level expression of IgH genes in myeloma cells can be maintained in the absence of the IgH enhancer. This has prompted us to propose an "establishment-only" model of enhancer function in these cells. This model suggests that the enhancer can introduce a stable change in a gene so that the enhancer becomes superfluous to the gene's continued transcription. An alternate explanation for these results is that additional enhancer(s) flank the IgH locus, substituting for the IgH enhancer when it has been deleted. In the pre-B cell, we have shown that an exogenously-provided IgH gene requires the enhancer both to establish its expression and to maintain it through successive cell generations. The difference between the results in pre-B cells and those with endogenous genes in myeloma cells may reflect differences between transfected and endogenous IgH genes or between pre-B and myeloma cells, or both. We will use the method of gene replacement by homologous recombination to test the "stable change" hypothesis. these experiments, it will be determined whether such a change can be effected in early stage lymphocytes and/or in transfected genes. Should the "flanking enhancer" model be favored by the results obtained, experiments will be undertaken to determine whether they are tissue-specific and what role they play in normal B lymphocyte development.